Most VTRs used for broadcasting are of 1-inch tape width or 2-inch tape width, and video signal recording system is such that a composite video signal is frequency modulated without any change. In recording/reproducing process, time base variation occurs due to the irregularity in the head rotation, the irregularity in tape travel or the like. This variation is corrected by a time base corrector (TBC) during reproduction by using horizontal sync signal or burst signal in the reproduced video signal. However, since the chrominance signal according to this system is subjected to quadrature modulation in NTSC system by subcarrier of 3.58 MHz to be superimposed upon luminance signal, when frequency modulated, it departs from its modulation carrier, and thus reduction of noise, which is a feature of FM, is not sufficient. Furthermore, chrominance subcarrier suffers from phase variation as much as residual jitter of TBC, and this becomes phase noise and thus the convergence of color vectors is insufficient.
As described in Japanese patent application No. 59-163155, there is a method for improving S/N of amplitude and phase direction of chrominance signal so as to improve the convergence in view of the above. According to this method two components of chrominance signal are also frequency modulated before recording, and time base correction is effected on playback before it is modulated (encoded) by a reference subcarrier to be added to luminance signal to form a composite video signal. According to this method since the chrominance signal (component signal) is also FM recorded at base band, reproduction is effected with good S/N. In addition, since encoding is effected with the reference subcarrier, there occurs no phase noise and thus it is possible to obtain satisfactory reproduced chrominance signal.
A conventional example of this system will be described with reference to FIG. 1. In FIG. 1, the references 1, 2 and 3 are input terminals for luminance signal Y, R-Y signal and B-Y signal; the reference 25, a sync signal generator; the reference 5, time base compressor; the references 4 and 6, frequency modulators; the references 7 and 8, heads, the references 9 and 10, frequency demodulator; the references 11 and 12, TBCs; the reference 14, reference signal input terminal; the reference 15, a sync generator; the reference 16, an encoder; the references 18, 19, 20 and 21, output terminals for Y signal, R-Y signal, B-Y signal and a composite video signal. The Y signal fed to the terminal 1 is modulated by the frequency modulator 4 to be recorded on a tape via the head 7. On the other hand, two chrominance signal components R-Y signal and B-Y signal fed to the terminals 2 and 3 are compressed by the time base compressor 5 in unit of 1 line so that time base becomes one half with a sync signal produced by the sync signal generator 25 using horizontal sync signal included in the Y signal being added to the R-Y signal. As a result, these signals are formed into a single signal such as R-Y.multidot.B-Y.multidot.R-Y.multidot.B-Y . . . (R-Y indicates R-Y signal compressed to 1/2 line) to be modulated by the frequency modulator 6 so as to be recorded on a tape via the head 8. Luminance signal and chrominance signal are recorded on a tape with different tracks being formed by the heads 7 and 8. Y signal reproduced by the head 7 on playback is demodulated by the frequency demodulator 9 and the time base thereof is corrected by the TBC 11. Chrominance signal reproduced by the head 8 is demodulated by the frequency demodulator 10, and the time base thereof is corrected by the TBC 12 and is expanded to its original length. The TBCs 11 and 12 writes a signal into a memory using a write clock generated using horizontal sync signal included in a reproduced and demodulated signal, and also performs time base correction and expansion by reading out signals from a memory using read out clock pulses which are generated by the sync generator 15 using a reference signal fed to the terminal 14. In addition, sync signal is removed, and a reference sync signal 24 generated by the sync generator 15 is added to the Y signal by the adder 13. In this way, it is changed to a sync signal having no noise so that reproduced Y, R-Y and B-Y signals are obtained at terminals 18, 19 and 20. On the other hand, the output, i.e. R-Y and B-Y signals, from the TBC 12 are encoded by the encoder 16 using a reference subcarrier 27 generated by the sync generator 15. Then these signals are added to Y signal by the adder 17 so that a reproduced composite video signal is obtained at terminal 21.
According to this system, since Y, R-Y and B-Y signals are used as inputs, when recording a composite video signal, these signals are led to input terminals 1, 2 and 3 after separation into Y, R-Y and B-Y signals by a decoder. On such separation, in order to provide wide band for the luminance signal and the chrominance signal, a comb filter using line correlation is generally employed.
The separation into luminance and chrominance signals using a comb filter results in the mixing of luminance signal into chrominance signal or vice versa in connection with portions where no correlation exists. The chrominance signal components R-Y and B-Y signals are added to a reproduced luminance signal after being converted into a carrier chrominance signal through modulation again by an encoder after reproduction. During modulation, when these signals are modulated by carrier having the same phase as the carrier chrominance signal which is not yet demodulated, to be added to the chrominance signal mixed into the luminance signal with the same phase, then the respectively mixed components are restored to their original states. If this is done, the high frequency components of the luminance signal are also transmitted after being mixed with the chrominance signal to be added to a signal transmitted in the luminance signal band with a right phase to obtain a satisfactory signal. In addition, the chrominance signal is also restored into its original state to obtain a signal having no color discrepancy or variation in saturation. However, the phase of chrominance subcarrier of the output signal from VTRs is generally arranged to be changed freely, in the TBC, with respect to the phase of the chrominance subcarrier of the reference signal from the input terminal 14 for the delay adjustment in connection with other video signal systems. Furthermore, while the phase of chrominance subcarrier of the input composite video signal on recording takes a round with four fields in the case of NTSC, the phase relationship between the reproduced signal and the reference input signal is usually only the decision of odd or even field. In such a case, the phase of the chrominance subcarrier to be modulated by the R-Y and B-Y signals and the phase of the original composite video signal are not determined. When modulated with a phase opposite to its original, the color at the portion where no correlation exists disappears, and the high frequency components of the luminance signal will disappear. Furthermore, unless the phases are completed matched, there occurs distortion in the high frequency range of the luminance signal and the chrominance signal.
Such phenomena would also occur to some extent when a low pass filter or a band pass filter is used without using a comb filter for the separation of the luminance and chrominance signal.
When dubbing a video signal with such a VTR, it is better that such dubbing is effected using a component output since the video signal necessarily passes through the luminance and chrominance separation circuit again when dubbing is effected with the video signal being converted into a composite video signal. In this case, a reproduced signal after several times of dubbing is finally encoded into a composite video signal. At this time, the same problem as described in the above would arise.